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FALL-OF-POTENTIAL METHOD TESTING PROCEDURE APPENDIX D: GROUNDING (EARTHING) ELECTRODE SYSTEM TESTING/VERIFICATION
4. Connect the Y lead to the measurement (Y) probe.
5. Determine the required placement of outer reference probe (Z) in accordance with paragraph
“Testing Area Requirements” on page D-5.
6. Place the outer reference probe (Z) into the soil at the determined location. Connect the outer
reference probe (Z) to meter terminal Z.
7. Note the distance of the outer reference probe (Z) from the grounding electrode system (this
distance will be used in determining subsequent measurements). On the photocopy of the Fall-of-
Potential Test Worksheet, write this distance in the “100%” space.
8. Place the measurement probe (Y) in the soil starting close to the area of the grounding electrode
system under test. Take a measurement.
9. On the photocopy of the Fall-of-Potential Test Worksheet (page D-21), write the reading in the
“0%” row of the worksheet.
10. Place the measurement probe (Y) at 5% of the distance between the grounding electrode system 'X'
connection and the outer reference probe (Z). Take a measurement.
Example: Assuming a “100%” distance of 131 m (430 ft.), Y probe would be placed at 6.55 m (21.5 ft.)
for “5%” measurement location.
NOTE: Placement of the measurement probe (Y) must be along a straight path between the grounding electrode
system 'X' connection and the outer reference probe (Z).
11. On the photocopy of the Fall-of-Potential Test Worksheet, write down the reading in the
appropriate row of the worksheet.
12. Place the measurement probe (Y) at 10% of the distance between the grounding electrode system
'X' connection and the outer reference probe (Z). Take a measurement.
Example: Assuming a “100%” distance of 131 m (430 ft.), Y probe would be placed at 13.1 m (43 ft.)
for “10%” measurement location.
13. On the photocopy of the Fall-of-Potential Test Worksheet (, write down the reading in the
appropriate row of the worksheet.
14. At every 10% point, repeat steps 12 and 13 for the remaining spaces on the worksheet.
15. On the photocopy of Fall-of-Potential Test Graph Form, plot a graph of the measured resistances
for all points listed on the worksheet.
16. Using Figure D-5, assess the test results as follows:
• Valid Test: At approximately 62% of the total distance (“100%” distance), a plateau or “flat
spot” should be noticeable in the plot, as shown in “VALID TEST GRAPH” in Figure D-5.
The resistance at this plateau is the validated resistance of the system under test.
• Invalid Test Graph (insufficient distance of Z-probe): If there is no plateau on the plot, shown
in “INVALID TEST GRAPH (Z-Probe Spacing)” in Figure D-5, the test is considered invalid
because the Z probe is not far enough from the X probe.
D-10 68P81089E50-B 9/1/05